pgsql: doc: Move force_parallel_mode to section for developer options

doc: Move force_parallel_mode to section for developer options

This GUC has always been classified as a planner option since its
introduction in 7c944bd, and was listed in postgresql.conf.sample.  As
this parameter exists for testing purposes, move it to the section
dedicated to developer parameters and hence remove it from
postgresql.conf.sample.  This will avoid any temptation to play with it
on production servers for users that should never really have to touch
this parameter.

The general description used for developer options is reworded a bit, to
take into account the inclusion of force_parallel_mode, per a suggestion
from Tom Lane.

Per discussion between Tom Lane, Bruce Momjian, Justin Pryzby, Bharath
Rupireddy and me.

Author: Justin Pryzby, Tom Lane
Discussion: https://postgr.es/m/20210403152402.GA8049@momjian.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ac725ee0f98c3fec703ffd9b070da629608e9a1e

Modified Files
--------------
doc/src/sgml/config.sgml                      | 99 +++++++++++++--------------
src/backend/utils/misc/guc.c                  |  4 +-
src/backend/utils/misc/postgresql.conf.sample |  1 -
3 files changed, 51 insertions(+), 53 deletions(-)